1. Field of the Invention
The present invention relates to a wavelength shift measuring apparatus capable of measuring a shift of a light source wavelength by being configured so that an optical path length difference of two light beams is constant.
2. Description of the Related Art
Conventionally, an interferometer has been used as a measuring apparatus for measuring a relative displacement or a refractive index of a machine stage. The interferometer generally measures with reference to a wavelength. Therefore, when a wavelength of the light source shifts, a measurement error is generated.
In this regard, conventionally, either (1) or (2) method has been used, i.e. (1) a configuration where an optical path length difference is set to zero is adopted so as not to generate an error caused by a light source wavelength shift in principle, or (2) a wavelength stabilization laser is used for suppressing a measurement error.
However, for example in order to suppress a measurement error in an interference measuring apparatus where an optical path length difference can not be set to zero for the intended use, there is no other way than to stabilize a wavelength. For example, Japanese Patent Laid-open No. 2002-319737 describes an optical communication light source where a wavelength stabilizing function is embedded. However, as described in Japanese Patent Laid-open No. 2003-202203, because the wavelength stabilization laser is expensive, the measuring apparatus can not be configured at low cost.
Japanese Patent Laid-open No. 2006-010499 discloses a configuration where a tilt is given to the optical path length difference by a wedge plate or the like and an interference pattern is generated by the interference of reflected lights on front and back surfaces. Further, it discloses that a light receiving element is disposed considering a bright and dark distribution of the interference pattern and that change of the bright and dark positions in accordance with the wavelength shift is detected by the fixed light receiving element. Japanese Patent Laid-open No. 2006-010499 proposes that the wavelength shift is detected with high resolution by the configuration.
However, commonly, a thickness of the wedge plate can not be accurate in a size around a wavelength. Therefore, the adjustment and the installation is necessary so as to be able to perform the most sensitive detection of the change of the bright and dark in accordance with the relation between the positions of the light receiving element and the interference pattern. On the other hand, such areas may be displaced in accordance with an external environment (temperature variation, application of vibration, or the like), and further stable method is necessary. In the wedge plate made of a glass or the like, because the optical path length difference of the front and back surfaces varies by the influence of heat expansion, a method for reducing the influence is also a problem.